This invention relates to making of high strength thin cast strip, and the method for making such cast strip by a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated, internally cooled casting rolls so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal is poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip to form a casting pool of molten metal, supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
In the past, high-strength low-carbon thin strip with yield strengths of 60 ksi (413 MPa) and higher, in strip thicknesses less than 3.0 mm, have been made by recovery annealing of cold rolled strip. Cold rolling was required to produce the desired thickness. The cold roll strip was then recovery annealed to improve the ductility without significantly reducing the strength. However, the final ductility of the resulting strip still was relatively low and the strip would not achieve total elongation levels over 6%, which is required for structural steels by building codes for structural components. Such recovery annealed cold rolled, low-carbon steel was generally suitable only for simple forming operations, e.g., roll forming and bending. To produce this steel strip with higher ductility was not technically feasible in these final strip thicknesses using the cold rolled and recovery annealed manufacturing route.
In the past, high strength, low carbon steel strip have also been made by microalloying with elements such as niobium, vanadium, titanium or molybdenum, and hot rolling to achieve the desired thickness and strength level. Such microalloying required expensive and high levels of niobium, vanadium, titanium or molybdenum and resulted in formation of a bainite-ferrite microstructure typically with 10 to 20% bainite. See U.S. Pat. No. 6,488,790. Alternately, the microstructure could be ferrite with 10-20% pearlite. Hot rolling the strip resulted in the partial precipitation of these alloying elements. As a result, relatively high alloying levels of the Nb, V, Ti or Mo elements were required to provide enough precipitation hardening of the predominately ferritic transformed microstructure to achieve the required strength levels. These high microalloying levels significantly raised the hot rolling loads needed and restricted the thickness range of the hot rolled strip that could be economically and practically produced. Such alloyed high strength strip could be directly used for galvanizing after pickling for the thicker end of the product range greater than 3 mm in thickness.
However, making of high strength, low carbon steel strip less than 3 mm in thickness with microalloying additions of Nb, V, Ti or Mo to the base steel chemistry was very difficult, particularly for wide strip due to the high rolling loads, and not always commercially feasible. For lower thicknesses of strip, cold rolling was required; however, the high strength of the hot rolled strip made such cold rolling difficult because of the high cold roll loadings required to reduce the thickness of the strip. These high alloying levels also considerably raised the recrystallization annealing temperature needed, requiring expensive to build and operate annealing lines capable of achieving the high annealing temperature needed for full recrystallization annealing of the cold rolled strip.
In short, the application of previously known microalloying practices with Ni, V, Ti or Mo elements to produce high strength thin strip could not be commercially produced economically because of the high alloying costs, difficulties with high rolling loads in hot rolling and cold rolling, and the high recrystallization annealing temperatures required.
The invention presently disclosed is a steel product comprised, by weight, of less than 0.25% carbon, between 0.2 and 2.0% manganese, between 0.05 and 0.5% silicon, less than 0.06% aluminum, and at least one element selected from the group consisting of titanium between about 0.01% and about 0.20%, niobium between about 0.01% and about 0.20%, molybdenum between about 0.05% and about 0.50%, and vanadium between about 0.01% and about 0.20%, and having a majority of the microstructure comprised of bainite and fine oxide particles containing silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers. The steel product may be further comprised of a more uniform distribution of microalloys through the microstructure than previously produced with conventional slab cast product. Alternatively, aluminum may be 0.008% or less by weight.
Alternatively or in addition, the low carbon steel product may have a total elongation greater than 6% or greater than 10%. The steel product may have yield strength of at least 55 ksi (380 MPa) or a tensile strength of at least 500 MPa, or both.
In addition, a thin cast strip is disclosed comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, and between about 0.01% and about 0.20% niobium, and having a microstructure comprised of a majority of bainite. The thin cast strip may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers. The steel product may be further comprised of a more uniform distribution of microalloys through the microstructure than previously produced with conventional slab cast product. Alternatively, aluminum may be 0.008% or less by weight.
The thin cast strip may a thickness less than 3 mm, or less than 2.5 mm, or less than 2 mm down to as thin as commercially feasible. The thin cast strip may have a thickness in the range from about 0.5 mm to about 2 mm. The thin cast strip may have a total elongation greater than 6% or greater than 10%. The steel product may have yield strength of at least 55 ksi (380 MPa) or a tensile strength of at least 500 MPa, or both.
In addition, a method is disclosed of preparing a thin cast steel strip comprising the steps of:                assembling a roll caster having laterally positioned casting rolls forming a nip between them, and forming a casting pool of molten low carbon steel supported on the casting rolls above the nip and confined adjacent the ends of the casting rolls by side dams,        counter rotating the casting rolls to solidify metal shells on the casting rolls as the rolls move through the casting pool;        forming from the metal shells downwardly through the nip between the casting rolls a steel strip; and        cooling the steel strip at a rate above 10° C. per second to produce a steel strip having a composition comprising by weight, less than 0.25% carbon, between 0.50 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, and between about 0.01% and about 0.20% niobium, and having a microstructure with a majority comprised of bainite. Alternatively, aluminum may be 0.008% or less by weight.        
The steel strip as coiled may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers.
The method of preparing a thin cast steel strip may further comprise the steps of: hot rolling the low carbon steel strip; and coiling the hot rolled low carbon steel strip at a temperature in the range from about 500-700° C.
The method of preparing a thin cast steel strip may also comprise the step of precipitation hardening the low carbon steel strip to increase the tensile strength at a temperature of at least 550° C.
The precipitation hardening may occur at a temperature between 650° C. and 800° C. or between 675° C. and 750° C.
The precipitation hardening may occur during the processing of the strip through a galvanizing line or continuous annealing line, or other heat treating process.